A triad is a three-note chord built by stacking two thirds on top of a root note: a root, a third, and a fifth.

Music Theory · Chapter 6, Lesson 2 — Triads & Chords

Building Triads in Thirds

Q: How do you build a triad by stacking thirds?

Pick a root note in a scale, then take every other scale note above it: the note a third up, and the note a third above that (a fifth above the root).

Q: What are the diatonic triads of a major scale?

Building a triad on each of the seven scale degrees of a major key gives this pattern of qualities: I major, ii minor, iii minor, IV major, V major, vi minor, vii diminished.

Q: Why is the triad on the 7th degree diminished?

The 7th degree's triad uses the scale's leading tone as its root, and stacking two thirds from there lands on notes that are only a diminished fifth apart, making it the one diminished triad in a major key.

Every chord starts somewhere — and for most of Western music, that somewhere is the triad: three notes, stacked in thirds, built straight out of a scale.

Intermediate ⏱ 6 min read 🎹 Interactive

What is a triad?

A triad is a three-note chord: a root, a note a third above it, and a note a fifth above the root (which is also a third above the middle note). "Stacking thirds" just means skipping every other note of a scale.

How to build a triad from a scale

Take the C major scale — C D E F G A B — and pick any note as your root. Then grab every other note above it:

Root: C
Skip D, take E (a third above C)
Skip F, take G (a third above E, a fifth above C)

C-E-G is the C major triad. Do the same starting from any other scale degree, and you get a whole family of chords that all belong naturally to the key — the diatonic triads.

🎹 Diatonic Triad Explorer Pick a key, then click any triad to hear it stacked from the scale.

Key

Notes of the clicked triad Key's tonic

The seven diatonic triads of a major key

Stacking thirds on every scale degree of a major key produces the same pattern of qualities every time — major, minor, minor, major, major, minor, diminished:

Degree	Roman numeral	Quality	In C major
1	I	Major	C-E-G
2	ii	Minor	D-F-A
3	iii	Minor	E-G-B
4	IV	Major	F-A-C
5	V	Major	G-B-D
6	vi	Minor	A-C-E
7	vii°	Diminished	B-D-F

Capital numerals (I, IV, V) mark major triads; lowercase (ii, iii, vi) mark minor triads; the little ° marks the one diminished triad, built on the leading tone.

💡 Why it matters: This exact pattern — major, minor, minor, major, major, minor, diminished — is the backbone of nearly every chord progression in Western pop, rock, and classical music, from I-IV-V to ii-V-I.

Common questions

What is a triad?

A three-note chord built by stacking two thirds on top of a root: a root, a third, and a fifth.

How do you build a triad by stacking thirds?

Pick a root in a scale, then take every other scale note above it — a third up, and a fifth above the root.

What are the diatonic triads of a major scale?

I major, ii minor, iii minor, IV major, V major, vi minor, vii° diminished — one triad per scale degree.

Why is the triad on the 7th degree diminished?

Its root is the leading tone, and stacking two thirds from there lands on notes only a diminished fifth apart — the one diminished triad in a major key.

Play every diatonic chord on your instrument

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Still stuck? Ask Your Personal Music Teacher about Building Triads in Thirds →

What is a triad?

How to build a triad from a scale

The seven diatonic triads of a major key

Common questions

Play every diatonic chord on your instrument

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